Containers currently in widespread use in the foodstuff industry include those made of aluminum or aluminum alloys, and those made of tin-plated ferrous metals.
Tin-plated ferrous metal containers tend to be relatively somewhat expensive due to the high cost of the tin-plate. A need therefore exists for relatively inexpensive unplated black plate steel containers, and such containers are presently under development. In order to form such containers, a process called "drawing and ironing" is used, wherein the steel sheeting is drawn and thinned to provide a cylindrical container of uniform wall thickness.
When the steel sheets used to form the containers are manufactured in the foundry, a protective oil such as cottonseed oil or dioctyl sebacate is placed on both sides of the steel sheets in order to protect the steel surfaces from abrasion when the sheet is rolled up for shipment to the container manufacturing plant.
When the rolled steel sheet is ready for use in the forming of containers, lubricants and other materials are coated onto the surfaces of the steel plates to facilitate the drawing and ironing operation. In particular, as the steel sheet is unrolled, the side of the steel sheet which is to become the inside of the container is coated with an acrylic wax, and the opposite side of the steel sheet, which is to become the outside of the container, is coated with molybdenum disulfide in an acrylic base.
These coating materials are necessary to facilitate the drawing and ironing operation. However, these coating materials are very difficult to remove from the surfaces of the containers, and conventional metal cleaning compositions, used for cleaning aluminum and tin-plated ferrous metal containers, have not proved suitable for removing these coating materials from the black plate steel containers. Removal of the mixture of molybdenum disulfide in the acrylic base has proved to be particularly difficult.
Commercial operations for cleaning containers prior to their further processing and filling with comestibles typically employ spraying operations. The metal containers are placed upside down on a conveyor belt, and the cleaning solution is sprayed into and on all surfaces of the containers. Following this cleaning step, the containers are sprayed with a rinse solution, usually clear tap water followed by deionized water, to remove remnants of the cleaning solution remaining on the surfaces.
Compositions have now been discovered that are useful in cleaning black plate steel containers when conventional spraying (or immersion) cleaning techniques are employed. Following the rinsing step, the containers are free of water breaks, showing that the containers are clean and free of the coating materials employed in their formation.
Prior art alkaline or neutral cleaning compositions for cleaning metal surfaces are disclosed in, e.g., the following patents:
U.S. Pat. No. 3,975,215 issued Aug. 17, 1976 to Edward A. Rodzewich PA1 U.S. Pat. No. 3,888,783 issued June 10, 1975 to Edward A. Rodzewich PA1 Japanese Pat. No. 53,100,131 issued to Asabi Kagaku Kogyo PA1 Japanese Pat. No. 53,149,130 issued to Nihon Parkerizing PA1 Japanese Pat. No. 53,045,309 issued to Nissan Motor PA1 U.S. Pat. No. 4,093,566 issued Dec. 27, 1976 to the United States Secretary of the Army PA1 (a) from about 20 to about 70% by weight, preferably from about 30 to about 50% by weight, of either an alkali metal metasilicate or an alkali metal orthosilicate, or a combination of such silicates. The silicate is preferably anhydrous, although the pentahydrate of the alkali metal metasilicate can also be employed. The alkali metal salts are preferably the sodium salts, although the potassium or lithium salts can also be used. Anhydrous sodium metasilicate is preferred since it is relatively inexpensive, is quite soluble in water, and provides high alkalinity and good detergency. PA1 (b) from 0 to about 45% by weight, preferably from about 5 to about 35% by weight of an anhydrous alkali metal carbonate, e.g. Na.sub.2 CO.sub.3 or K.sub.2 CO.sub.3. A low density alkali metal carbonate, e.g. low density sodium carbonate, is preferred. PA1 (c) from about 5 to about 30% by weight, preferably from about 10 to about 25% by weight, of an anhydrous condensed alkali metal phosphate. The alkali metal phosphate can be either an alkali metal tripolyphosphate, a tetra alkali metal pyrophosphate, an alkali metal hexametaphosphate, or a combination of one or more of the above. PA1 (d) from about 2 to about 15% by weight, preferably from about 3 to about 6% by weight of a compound of the formula ##STR1## wherein R is a saturated C.sub.8 -C.sub.12 alkyl group, and x is in the range of 8 to 12, and having a cloud point (1%) in the range of from about 45.degree. to about 65.degree. C. The C.sub.8-12 saturated alkyl group can be, e.g. octyl, nonyl, or dodecyl, with nonyl preferred. The preferred compound is nonylphenoxy-(polyethoxy).sub.9-10 ethanol, having a cloud point of about 54.degree. C., sold commercially under the trade name "TRITON N-101" by Rohm & Haas Company, "TERGITOL NP-9" by Union Carbide, and "IGEPAL CO-630" by GAF Corporation. PA1 (e) from about 2 to about 20% by weight, preferably from about 4.5 to about 12% by weight of a polyethoxy secondary alcohol of the formula EQU C.sub.12-14 H.sub.25-29 O(CH.sub.2 CH.sub.2 O).sub.x [CH.sub.2 CH.sub.2 O/CH.sub.2 CH(CH.sub.3)O].sub.y CH.sub.2 CH(CH.sub.3)OH, PA1 wherein x is in the range of from about 5 to about 9, preferably about 6 to about 8, y is in the range of from about 1 to about 5, preferably about 2 to about 4, and the cloud point is in the range of about 35.degree. to about 45.degree. C. The preferred compound for use in the practice of the invention has a molecular weight of about 706, and is sold commercially under the trade name "TERGITOL MIN-FOAM 1X" by the Union Carbide Corporation.